The broad objective of the proposed research is to elucidate the metabolic control mechanisms which regulate the synthesis of one neurotransmitter, gamma-aminobutyric acid (GABA). The present evidence indicates that glutamate decarboxylase (GAD), the enzyme responsible for GABA synthesis, may be regulated in part by an interaction between the pyridoxal-5'-phosphate (cofactor of the enzyme) and nucleotides (e.g. ATP and ADP). This concept is based on kinetic studies of the enzyme and is supported by the observation of a postmortem increase in the degree of saturation of GAD by pyridoxal-P in the intact animal. Other evidence indicates that GAD exists in both high- and low-molecular weight forms. In addition, it has been suggested that GABA feeds back to inhibit pyridoxal kinase, the enzyme responsible for the synthesis of pyridoxal-P. The proposed studies are designed to attack this problem two ways: first, by detailed studies of the regulatory properties of the purified enzymes, and second, by studies of the regulation of GAD in brain slices and in the intact animal. Studies of pyridoxal kinase will deal primarily with the mechanism of inhibition of the enzyme by GABA in an effort to determine whether this inhibition by GABA can take place in vivo. Studies in brain slices and intact animals will be designed to test possible regulatory mechanisms deduced from the enzyme studies to determine if changes in energy metabolism are reflected in changes in the degree of saturation of GAD.